Interactive computer graphics is the most important means of producing pictures since the invention of photography and television. With the aid of a computer, digital graphic images can be edited to create a variety of effects not easily obtainable with conventional graphic editing techniques.
A very common and powerful digital graphic editing operation is compositing, which involves overlaying a cutout feature on a background image to form a composite image. In order to blend the cutout feature with the background image, it is often desirable to soften the edge of the cutout feature to provide a smooth transition between the cutout feature and the background image. For images stored in the RGB.alpha. format, edge softening is often achieved by modifying the alpha values (which indicate degrees of transparency or opacity) of pixels of the cutout feature near its edge to make those pixels more transparent. As a result, the background image pixels under the cutout feature become partially visible, thereby creating a visual effect of a softened edge of the cutout feature.
The alpha value modification for edge softening is typically performed by blurring the alpha mask associated with the cutout feature around the edge of the cutout feature. The blurring operation involves a convolving process which uses a blur mask with a pre-selected blurring width. Because the convolving process involves many pixels, edge softening by blurring is calculation-intensive and is therefore relatively slow. The slow response makes iterative graphic editing a time consuming process and thus impacts negatively on the interactive aspect of the computer graphic editing application. If the user is not satisfied with the result of edge softening with a previously selected blurring width and wants to try a different blurring width, another complete convolving operation has to be performed. Due to the long computational time required for convolving, the resultant composite image may not be ready for viewing in real time.